Invasive Species of Aquatic Plants

Potamogeton crispus
(curly-leaf pondweed)
can be identified by its wavy, green or reddish-green leaves (diagram courtesy of USGS.

A diverse community of aquatic plants is generally recommended to suppress phytoplankton, support fish and provide a number of other functions important to pond water quality. In some instances, however, invasive species may take over and impair pond function. Pond owners are advised to attempt their removal before they become well established. Three species likely to colonize ponds in southeast Pennsylvania are described below.

Curly-leaf pondweed (Potamogeton crispus) is an aggressive underwater plant, originally from Europe, which has become well established in ponds and streams of Chester County.

Whereas most aquatic plants germinate and begin to grow actively in spring or early summer, completing their life cycle in fall, curly-leaf pondweed germinates in fall, grows rapidly during early spring, setting seed and decomposing by early July. Because of its unusual life cycle, it helps to control phytoplankton by taking up nutrients and regulating water movement in spring, but may stimulate phytoplankton growth when it senesces in mid-summer.

Eurasian water milfoil
can be differentiated from other native milfoils by its very “feathery” leaves with long filiform extensions. Diagrams courtesy of the University of Florida, Center for Aquatic and Invasive Plants.

Eurasian water milfoil (Myriophyllum spicatum) is a submersed, rooted species with deeply divided, featherlike leaves usually arranged in whorls of four (range 3-6). It can be distinguished from other aquatic milfoils by the higher number of filiform extensions (14-24) on each side of the central leaf axis. Like curly-leaf Pondweed, Eurasian water milfoil propagates rapidly, tolerates low light levels and is an effective competitor for nutrients. Based on a survey of 50 ponds in Chester County during summer 2003, Eurasian water milfoil appears to be rare in Chester County, but is likely to be a threat to ponds in the region in the future.

Hydrilla
is a highly invasive exotic, distinguished from the similar native species Elodea canadensis by the larger number of toothed leaves per whorl. Diagrams courtesy of the University of Florida, Center for Aquatic and Invasive Plants.

Hydrilla (Hydrilla verticillata) is also a submersed, rooted species with long stems typically reaching the surface. Relatively rare in this region, it can be distinguished from Elodea (Elodea canadensis, a similar but native species commonly found in ponds in Chester County), by the presence of teeth on the leaf margins and underside of the midrib, and by the larger number of leaves per whorl on the stem (Hydrilla has 4-8; Elodea typically has 3).

Water Chestnut (Trapa natans) is native to Eurasia, but has become an aggressive pest in lakes and ponds of
the northeastern U.S. and has recently arrived in Pennsylvania. An “annual” (with a lifespan of just a single
growing season), Water Chestnut nonetheless proliferates rapidly both by copious seed production and
vegetative spread (by breakage and the dispersal of leaves and stems). At high densities the plant may
cover the water surface, blocking light penetration and preventing the establishment of other species in
the water below. Mechanical removal (see photo below) is the preferred method of eradication, and also
removes the nitrogen and phosphorus stored by the plants, thereby reducing pond nutrient levels.

Extracting Water Chestnut plants from a Chester County Pond.

Dispersal of these invasive species likely occurs without human assistance (e.g., by the movements of waterfowl). If one of them does become established, an aggressive, multifaceted management plan to
eliminate it or greatly reduce its abundance is warranted. Such a plan might include a combination of
herbicides,
drawdowns
or mechanical removal. Use of specialist aquatic insect herbivores as biological controls may also soon become feasible based on promising current research. Sadly, unless completely removed, the plants are likely to require continued management efforts.